Among heretofore used methods for the diagnosis of rheumatoid arthritis may be mentioned: Rose's method based on the detection of rheumatoid factor, modified Rose's method by Heller, RAHA-test and RA-test. These methods, however, have such disadvantages that they use complex experimental methods or that it takes many days to go through with the diagnosis.
Collagenase inhibitor has been known to exist in human's and other animals' bones, skins, dental pulps, amniotic fluids, bloods and synovial fluids as well as in cultures of joint chondrocytes, synovial cells, fibroblasts derived from varied tissues and fibrosarcomatous cells. As means for determining the amount of collagenase inhibitor methods have heretofore been known which are based on measurement of its biological activity. As described by Eisen et al. in J. Lab. Clin. Med. 75, 258-263 (1973) and also by Cawston et al. in Arthritis and Rheumatism 27, 285-290 (1984), however, it is impossible, with the heretofore known methods of determination, to assay sera, plasmas or synovial fluids for collagenase inhibitor activity since there exist in such fluids proteins, such as .alpha..sub.2 -macroglobulin, which interfere disturbingly with the measurement. Hayakawa, Iwata et al. have already developed a sandwich enzyme immunoassay (EIA) using monoclonal antibodies to bovine collagenase inhibitor, a method which enables specific quantitation of collagenase inhibitor with very small volumes of samples in a precise, straightforward and rapid manner (Japanese Patent Application No. Sho 62-42781). The present inventors have discovered that the level of collagenase inhibitor present in human sera, plasmas or synovial fluids apparently increases in association with affliction with rheumatoid arthritis, and found that it is possible to make a diagnosis of rheumatoid arthritis by determining, by the enzyme immunoassay mentioned above, the amount of collagenase inhibitor present in sera, plasmas or synovial fluids.